


Twins Are Dangerous: Demon Training

by theflyingpan



Series: Twins_Are_Dangerous_AU [6]
Category: Fire Emblem Series, Fire Emblem: Fuukasetsugetsu | Fire Emblem: Three Houses
Genre: I only tagged those who spoke, I wrote dis, Twins_Are_Dangerous_AU, but there are mentions of more characters, so.many.characters.
Language: English
Status: Completed
Published: 2019-10-15
Updated: 2019-10-15
Packaged: 2020-12-16 18:29:07
Rating: General Audiences
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 5,043
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/21040778
Author URL: https://archiveofourown.org/users/theflyingpan/pseuds/theflyingpan
Summary: Weeks after what most of the students at Garreg Mac refer to as "The Incident", the Byleth twins approach their father and ask him to spar with them. Sothis is just glad the two aren't trying to kill each other from stress. Rhea is glad they told her before fighting. The students are always down for a good fight.(Unofficial sequel to "Twins are Dangerous: An aside by a Sibling")(I wrote this one not my sister though)





	Twins Are Dangerous: Demon Training

**Author's Note:**

> The unofficial sequel to "Twins are Dangerous: An aside from a Sibling" from your regularly scheduled author!

Sothis gazed as the twin Byleths stood in their defensive stances, side by side, stands mirroring each other. Unlike what Sothis and the students refer to as “The Incident”, the twins were facing not each other in the training grounds, but their father. Jeralt was holding a steel lance, standing casually and staring at his kids with the same blank stare they gave him. The male toyed with the training sword in his hands, twisting the weapon around, while his sister held hers in a tight grip. The Sword of the Creator was with Jeralt, safely tucked between his back and his shield—wrapped in a cloth for protection at his daughter’s insistence— and Sothis floated behind him to ensure it was securely tightened to his body. She then floated around and looked around the room at the audience that gathered. This time around, the twins were much more calm and controlled. If the nobles were still causing them stress and frustration, they didn’t let it show when they asked their father to train with them in their free time. He agreed, relieved that he wouldn’t have to walk in on another death match, and the three decided to give Lady Rhea a bit of a warning this time around—since she seemed pretty upset over “The Incident”. The four walked to the grounds together, the two adults flanking the twins— which is how Leonie and Marianne found them. Leonie all but demanded to know what the four (but mostly Jeralt) were planning, and as Jeralt explained Marianne kept her eyes trained on the ground, unable to bring herself to meet the Archbishop’s eyes.

“It’s an 'Ashen Demon' training session” Byleth added as his father finished the explanation, and Lady Rhea frowned at the nickname while his sister rolled her eyes. Leonie’s eyes widened in excitement as she recalled the last time the twins trained, and the female professor had to move rather quickly to catch Marianne when she also recalled “The Incident” and fainted as a result. Leonie carried Marianne back to her room, all the while spreading the news that yes, the professors were going to battle again, and no, this time they won’t be killing each other. Soon, a good deal of students had gathered at the training grounds, just in time to see the twins pick out a training sword each and Jeralt get geared up. Lady Rhea stood off to the side as a referee, her hands joined at the middle and ready to heal any injuries if necessary. Sothis floated beside the woman, despite her apprehension, and the twins looked to them and waited for a signal.

“You may begin” Rhea called out just as Sothis raised and lowered her arm in her usual confirmation. Jeralt turned to face his kids and waited for the first move. His son struck first, thrusting his sword at his father and quickly turning the stab into a slice. Jeralt recognized the move from when he battled with Catherine, and moved to block both attacks with ease. His daughter was a blur behind him, and he flicked the lance to block the downward sweep of her blade, like he would when training with Alois and his axe. He soon relaxed in the movements of blocking the attacks his kids used, remembering the times he would do this years ago as the twins first started to learn to use their blades.

“Wow!” Leonie gasped, eyes wide as she watched the three dance in a dangerous battle. “Jeralt is moving so effortlessly!” Lysithea rolled her eyes.

“He’s probably trained them since they were _children_” She sighed, glancing up from the book she brought to the battle, watching as Byleth rolled out of range of an attack that nearly sliced her hair. “It’s no wonder that he can easily move with them”. Ignatz stared wide eyed at the three.

“Still, it’s amazing how well they work together” He mumbled as Jeralt knocked his son back a few feet with a hard kick. Caspar yelled in excitement at that.

“Go Captain!” He cheered, causing Linhardt to scoff.

“I don’t think you’re _supposed_ to cheer for anyone Caspar” Linhardt sighed, rolling his eyes. “It’s _just_ a training session.”

“It is kinda hard to not cheer for anyone though” Ferdinand commented as the twins sliced at their father in sync, blades clashing against the hard metal. Petra was silent in shock, but managed to nod in agreement. Sothis brought her attention back to the battle, trying to tune out the student’s chatter. Byleth was panting in frustration, and his sister was debating tying her hair up in order to attack more efficiently. Their father was barely out of breath, and smiled at his kids as they continued to attack. Jeralt sighed playfully as he blocked another attack from behind.

“You guys have a good grasp on the new techniques you’ve learned here, don’t you?” He asked as he effortlessly swept his lance at his son’s feet, who jumped back in response. His daughter hummed in agreement as she was knocked off her feet from the same sweep of the lance. “You two must really be dedicated to these kids, if you’re learning moves from the other faculty here, and actually _using_ them.” He pushed them both away as they swung down on the lance, getting the correct amount of distance for the next stage in their training. He let the dull end of his lance hit the dirt, and the twins slowly straightened up, understanding his body language as they awaited his new instructions. Jeralt took the time to glance around the room and inspect the crowd.

The Blue Lions were cheering at the display of power, always itching for a battle, and the Black Eagles were silently taking notes on the way the twins moved. The Golden Deer class was just as loud and enthusiastic as the Lions, but Jeralt suspected that was mostly because they wanted to out-yell the other classes. He also spotted the three house leaders, standing behind Lady Rhea and staring intensely at the three of them. Claude, Dimitri, and Edelgard were watching their professors move in silence, from either awe terror or joy was hard for Jeralt to determine. They also stood near the entrance in case they needed a quick getaway, but after catching Jeralt’s eyes the three shuffled a bit away from the doors. He smiled and turned back to his kids, moving into an attacking stance.

“You guys haven’t practiced your 'demon' techniques in a while, right?” He asked, and his son’s eyes lit up. “You talked big talk to Leonie about this 'Ashen Demon' training session, but let's see if you still have the ability to use those tactics” Byleth grinned a bit before standing up straighter, and his sister backed up to allow her brother to go first.

“So…what’s going on now..?” Sylvain wondered out loud, as the female twin stood next to Lady Rhea and her brother stood in the middle of the training grounds. Felix ignored him in favor of watching his professor, who was standing in a way that was so familiar to him yet Felix had never actually seen it before. His professor had his weight on one leg, the other slightly bent at the knee, and had tilted his head to give his father a smug smirk--one that Felix had worn when he first started training. His father and other instructors had long beaten that cocky attitude out of him; but Felix was still excited to see his professor act as smug as he once did, knowing that his professor had a way of turning everything that the knights taught on its head and revealing _actual_ useful fighting techniques. Claude let out a low whistle and leaned further to get a closer look at Byleth in the ring, causing Dimitri and Edelgard to give him a confused look.

“I think I know where this is going~” He grinned, and missed the way that the female professor glanced over at him with a strange expression. Byleth let out a deep breath and closed his eyes, allowing himself to put on the face he wore for years before entering the monastery. He opened his eyes again, half lidded and gazing blankly at his father with just the slightest smirk on his face. The look sent shivers down both Rhea’s and Sothis’ spines. It was the face of the Ashen Demon.

Byleth stood still with one leg behind him, holding his sword at his side with the blade pointed downwards, and Jeralt started a basic lance attack. It was a simple move that beginners often used in battle: get close enough to the enemy by either lunging or jogging towards them, then stop your approach and thrust your lance outwards, hopefully lodging the tip of your weapon in the enemy’s chest. Jeralt started to move towards him, faster than any newbie might, and Sothis heard Rhea suck in a breath of fear as Byleth continued to stay still. Rhea started to raise her arm, ready to halt the match, but another hand stopped the action by placing grabbing her arm. Rhea stared at the female twin, who gave her a hard stare before letting go of the archbishop’s arm, re-focusing on her brother and father. Rhea frowned at this but put her hands back to rest at her middle, trusting the woman enough to let the match continue. Jeralt got close enough to his son to stop his forward march, and Byleth let his small smirk grow a bit wider at this. As Jeralt took a step to ground himself and ready his thrust, Byleth shifted his feet backwards and threw his sword high in the air, causing Sothis to gasp a bit and lean on his sister’s shoulders to get a closer look. Jeralt thrust at his son, and Byleth let the lance get close enough to barely touch him before he ducked under the attack and jumped up to catch his sword as it started to fall. He plucked the weapon in the air and twisted his body, using the momentum of the fall to hit his father’s shield on his back. He landed on his dad’s back as the weapon struck and snapped clean in two, the broken blade flying out and hit the wall next to poor Marianne, who promptly fainted and fell over (Sothis was _really_ concerned about the girl’s bad luck) at Hilda’s feet. Byleth stayed on his father’s back for a moment, and the sound caused by his sword continuing to ring out in the now silent grounds. Jeralt let him stay on before tapping his son’s feet with a small grunt, and Byleth slipped off his father's back and gently land on the ground.

Caspar was the first student to make a sound, babbling nonsense as his brain exploded from the sheer awesomeness that he just witnessed(At least, that’s what he said hours later when he could finally process what happened). Claude let out another low whistle and smiled at his professor as the rest of the students slowly got out of their shocked states. Felix, once he got over the initial surprise, was fighting against Mercedes and Annette as the females tried to prevent him from jumping into the ring to try that move on the professor himself(“Let me have a go! I can do it no problem, I want to try!!”), Leonie also fighting against her fellow classmates who prevented her from begging Jeralt to teach her the lance attack he used(“Captain! Captain Jeralt! What was that move called?? Did you make that up?? Can I try?!”). Lorentz took it upon himself to correct his professor’s body language(“_I_ would _never_ risk throwing a weapon like _that_! You’re practically _defenseless_ in front of the _enemy_!”) and Ferdinand slowly moved to cover the purple-haired man’s mouth so he could focus on the scene he just saw properly. The female twin at Sothis’ side simply crossed her arms and looked at her brother as if she was disappointed, watching as Jeralt let out a sharp laugh and stood up straight, turning to face his son with a kind smile.

“That was pretty good, but you’re getting slower” He commented, waving his daughter to join them in the middle of the ring, who simply nodded to agree with her father. Sylvain coughed to cover up Felix’s sharp laugher at this (“I bet I can move faster than him! Let me try!!”) and Ingrid raised an eyebrow.

“That seemed pretty fast to me” She murmured as Jeralt gently corrected his son’s stance (“You threw your sword too high, and you ducked down too low”). Claude grinned at his fellow house leaders, nearly losing his composure at Dimitri’s flushed face.

“How did you guys like that~?” He hummed. “The Throw of the Ashen Demon’ is what my people called it” Edelgard raised an eyebrow at this.

“_Your_ people?? What, have they seen that move before?” She frowned a bit and jabbed a finger at him. “Have _you_ seen this move before?” Claude laughed off her suspicions and rolled his eyes, batting her hand away and missing the sudden turn of Hubert’s face as he snapped his head to turn and glare at the noble for even daring to touch Lady Edelgard, let alone hit her in such a way(Thankfully Hilda managed to distract him and save Claude’s life…for now).

“Teach was a _mercenary_, remember? The three of them traveled all over the place, I’m surprised you didn’t see his move before” He smiled and turned his attention back to the twins. Edelgard rolled her eyes.

“Of course I’ve heard of the ‘Ashen Demon’ and the fighting techniques— _that_ wasn’t it though” She huffed, crossing her arms and following his gaze. Her house professor was up next, and Byleth’s father helped her brother into a stronger armor set for his chest as she grabbed a sheath for her sword(which was rather hard to find, considering that most of the students went without one for some reason). Jeralt also moved his shield from his back to wield in both hands, tossing the lance behind him and catching Sothis’ attention. She floated to stay behind the older man, keeping a close eye on the now exposed Sword of the Creator. Dimitri finally snapped out of his daze and leaned against his peers standing in front of him as he took in the scene.

“Wait, are they _both_ going to face Miss?” He asked no-one in particular, and Lady Rhea turned to glance at the lords behind her. She gave them what she hoped to be a reassuring smile before turning back to the three in the ring, a frown twitching on her face. Byleth stood farther away from her opponents than her brother had, and too she closed her eyes as she mentally put herself back in the shoes of her younger self—the once expressionless and cold hearted Ashen Demon. As she opened her eyes to give her father and brother a dull and blank expression, Jeralt clapped a hand on his son’s back to hold him steady as his other hand moved to hold the shield in front of him. He gave his daughter a nod, signaling to her that they were prepared, and the noise that Byleth’s attack against his father sparked suddenly died down as everyone stared at the female professor in anticipation. Unlike her brother, Byleth stood at her full height, her face devoid of any emotion, eyes betraying nothing, slowly swaying left and right. Her sword was sheathed and held loosely in her left hand, blade pointed down as she blinked slowly at her opponents, making Sothis shudder as Edelgard drew in a sharp breath.

“This is what we called the ‘Strike of the Ashen Demon’” She murmured, and she leaned in closer next to Claude. “_My_ people thought she was _possessed_” That’s when Byleth started her attack.

It all happened in a flash. Byleth slowly shifted her center of gravity, allowing herself to gently fall to her left. Dorothea gasped a bit as Byleth let herself fall nearly sideways, before her right leg snapped forward to prevent herself from hitting the ground. Her left leg shifted behind her and she all but lunged at her brother and father, who merely gripped the shield tighter and ducked their heads a bit. The momentum from her fall allowed her to lunge forward in a large arch, the crest being where she grabbed the handle of her blade with her right hand and swung it to connect with her father and brother. Jeralt grunted as he fought against the force of his daughter’s attack, struggling to keep himself and his son standing upright as she sliced her blade at them. Byleth completed the arch and returned to her origin, throwing her left hand at the ground to slow her down and stop her from falling forward as her right hand with the sword stayed stretched outwards to preserve her balance. She stayed like that for a moment as she caught her breath, with her face close to the ground and her long hair gently touching the sand—her stance making her look like some sort of demonic _beast_—before she stood back up, her sword having found its way back into the sheath in the process. She brushed the hair away from her face and slowly closed her eyes again, sighing a bit as she let her mind shift back from the Ashen Demon to the friendly professor. The room was deadly silent, until Jeralt broke the silence with a sigh.

Caspar fainted from the(again, his words) sheer awesomeness that he just witnessed as the room exploded with just plain _noise_ as Jeralt unclasped the armor from his son’s body.

“You’re falling too far” He commented, waving his daughter over to show her the damage on the chest plate. There was a large and deep gash were her blade connected, and even Linhardt had perked up and paid attention at this. “You got your brother pretty good, but my shield barely has a scratch” Jeralt continued and held his shield up for his daughter to compare. Byleth ignored her brother’s teasing grin and pressed a hand on the shield, feeling for any marks.

“…I didn’t want to damage it” She explained, looking at her father. “You don’t have any spares with this design, so I’d hate to break it” Byleth coughed a bit and avoided his sister’s hard stare as Ashe’s jaw finally relaxed enough for him to pick it up from the ground.

“D-does that mean that M-Miss was holding back??” He asked frantically, glancing between Ingrid and Dedue. “She could’ve broken _that_ shield!? In _half_??” Ingrid was just as shocked at this revelation, and Dedue simply shrugged.

“It would appear as if that were the case” He replied, glancing over at Mercedes, who was fanning a fainted Annette with a calm smile. Hubert huffed from his place next to Dedue and rolled his visible eye.

“Even if she wasn’t, she’s still not nearly as powerful as her brother. He managed to break his sword, but she barely made a scratch in her own” He commented, not expecting Bernadetta to speak up from her hiding place behind Raphael.

“A-a-actually” She mumbled, peeking out long enough to see her professor glance around and make eye contact with her. “doesn’t that mean she’s the stronger one..?” Hubert glared at her for even daring to oppose him(even if in a casual conversation) and Bernadetta yelped and hid behind Raphael again. “I-I-I mean she was in more control than her brother!” Hilda hummed at this from her position on the floor, trying to coax Marianne back to health.

“That’s true. I guess that Teach is more unhinged and forceful, while the Prof is more calculating in her attacks” She shrugged, and even Lysithea looked up from her book at the sound of Hilda actually putting effort into her thoughts. Sothis had to giggle at this, and as Jeralt placed his damage free shield back to cover the Sword of the Creator, she allowed herself to relax and float back to the three nobles— assessing their reactions.

If Dimitri’s blood started to boil at the sight of the male Byleth’s attack, then his blood was simply melting in his veins when the female twin completed her own. He tried to wipe away the grin that found its way on his face, and leaned against one of the columns behind the other two nobles. Edelgard was smiling at Claude, who was struggling to not let his jaw drop at the sheer power that Byleth displayed.

“So, you heard _all_ about the Ashen Demon, didnt you?” She teased, happy that she finally got to see the Strike of the Ashen Demon she heard so many rumors about. Claude waved at her with his hand, trying not to let her words get to him.

“Yeah yeah okay, well maybe I didn’t hear the _whole_ story” He countered, turning to face her again. “Not _everyone_ knew the Ashen Demon was actually a pair of twins after all, we only ever saw _one_ of them in a battle”. His comment brought Dimitri back into the conversation.

“Actually, most people in my homeland knew that.” He mentioned, idly fixing his blue cape as Edelgard and Claude slowly turned to face him in surprise. Dimitri blankly stared at them in confusion, dropping the cape in his hands. “…what? You guys didn’t know?” A beat of silence. “The Ashen Demons? Siblings who wore the same face? Who haunted the earth and drank the blood of their enemies? Who you had to defeat in one move, at the exact same time, lest the one who survived killed you in the most gruesome way possible? Twins who attacked at the same time, and who never left each other’s sides? Capable of killing twice as many men twice as fast? You guys didn’t have those rumors?” At this point, even Lady Rhea had turned to give Dimitri a surprised look. He blinked blankly at this, gazing back at the twins in the ring. Jeralt had grabbed one of the humanoid training dummies, and the male twin had replaced his broken sword. “Ah, here, watch this: the Onslaught of the Ashen Demons” He pointed to the ring, and the other nobles turned to see Jeralt kick his lance farther away and back up from his kids.

The Byleth twins were standing side by side once again, swords in their hands, as they faced their father with their Ashen Demon expressions. He tossed the wooden training dummy at them (that was supposed to be a stationary opponent, Sothis noted with a sigh), and his son caught it with ease. He threw the dummy onto his leg as he kicked the body up, and his sister jumped a bit to use her sword to knock it back down to the ground. Byleth then grabbed the dummy as it bounced from the ground, kicking it up again. The twins did this a few more times, slowly knocking wooden chips off the dummy and creating small yet deep gashes in the body, before Byleth moved to stand behind her brother a few feet. Her brother grabbed the dummy again and threw it behind him without looking, as Byleth swung her sword sideways like an axe to hit the dummy clean in the middle, slicing the thing in two.

“…oops” She muttered, and her brother turned to see the damage before wincing a bit. The entire training grounds was silent, and the silence didn’t break even as the bells rang a moment later. Lady Rhea was the first to recover, gently clearing her throat and walking towards the middle of the grounds.

“I _believe_ that those bells signaled the start of the next lecture, don’t you?” She announced gently, and the students shook themselves out of their daze and scrambled to get to their classes on time. Jeralt busied himself with cleaning up the weapons, and the twins took the two ends of the training dummy and attempted to find a way to fix it, sharing a confused and worried look. The four didn’t notice the three leaders standing still, not moving a muscle, and Sothis floated over to try and find out why they were so immobile.

“…Yep, that’s them.” Dimitri finally said, shaking his head and trying to ignore the way his body begged him to walk to the twins and demand a battle. He smiled at the other two nobles and turned to walk to his class. “The Onslaught of the Ashen Demons” He hummed on his way out. Claude shook his head and turned to follow.

“That’s our professor, I guess,” he mused on his way out. “Answers one question and raising five more”. Edelgard lingered behind, watching the twins hand over the broken items to their father as Lady Rhea watched them with a concerned look.

“…yeah…Demons…” She murmured, and turned to walk off. Sothis watched their retreating backs before slowly returning to the twins, just in time for her to hear Rhea speak again.

“Those techniques were really..._interesting_” Lady Rhea started, a thin smile on her face. Byleth spared a glance at his sister, before Jeralt sighed a bit and nodded.

“Well…I suppose so.” He shrugged and took the undamaged sword and sheath from his daughter. Rhea’s eyes shifted to the broken sword in Byleth’s hands, watching the way his fingers curled just a bit around the blade, before looking back at Jeralt.

“They were also really _dangerous_, don’t you think?” She asked carefully, and Jeralt had to suppress another sigh. After developing the moves with the twins and having them successfully perform them in a battle, he was used to being asked this over and over.

“We developed it together, the three of us” Jeralt started to explain, finishing cleaning up and turning to face the archbishop. He noted the way her smile was gone, and the way she looked at him with upset eyes. He waved his kids away, and they walked off to start their lectures. They avoided it now, but he could only imagine all of the pestering questions about their “Ashen Demon” techniques and how they managed to perform them that their students would ask them once they saw the twins again. He looked back at Rhea’s judgmental face before continuing to explain. “Byleth has little to no care about his own health, so we used that when developing that move to avoid attacks. He can also use it to disarm enemies.” The two started their way back to Rhea’s office, and Jeralt nodded at some of the passing students as Rhea waved kindly. “...I also don’t like how it requires him to get so close to danger, but he used to do something similar in battles before we developed that. At least the way he does it now, it’s less dangerous to him and he is actually successful that way.” Rhea seemed to relax at this, and they ascended the stairs in relative silence.

“And his sister’s move?” She asked gently, surprising Jeralt with her interest in his daughter. He scratched the back of his head and shrugged a bit.

“I guess it’s the same idea…She used to get dizzy spells when she was younger, but every time she nearly fell to the ground, she managed to stand right back up.” Jeralt smiled a bit as he remembered the way he and his son would panic when Byleth seemed to fall, only to be shocked as she suddenly straightened up, her other leg snapping to ensure she didn’t hit the ground. “It was a simple task to just hand her a sword and challenge her to make it into an attack.” Jeralt chucked a bit as he then thought of the times Byleth would fall when she timed the fall wrong, or how she misstepped and fell on her face when she got her footing wrong. In fact, she fell over _more_ when she was perfecting her move than from her dizzy spells. Rhea hummed as she took the information in, and the two walked into her office quietly.

“But can they perform each other’s moves?” She pressed, and Jeralt sighed a bit.

“In theory? Yes, they could. But Byleth always seems to move too quickly and throw the sword too low when she tried, and Byleth never gets the timing right for the fall.” Rhea nodded at this, looking at the papers on her desk idly before turning to face Jeralt again.

“Okay…but what about the synchronized move?” She turned and faced him fully, hands at her middle. “That attack seems like it’s less offensive and more defensive.” Jeralt nodded at this, crossing his arms and gazing to his side.

“Yeah…it’s more of a move they use when they’re cornered or facing multiple enemies. Byleth can just keep grabbing and tossing enemies to his sister, and she can continue slicing or bashing for a long period of time” Rhea thought for a bit quietly and nodded, finally satisfied with the answers.

“...You did a relatively good job at keeping them safe, despite your job as a mercenary, didn’t you?” She smiled, and Jeralt could see the thinly veiled sarcasm in her voice. He decided to return it.

“Well, it was either teach them how to use their weaknesses as strengths, or let them just sit still and get themselves killed. Which would be a shame considering _all_ the hard work I did to get them this far.” He mused, smiling as Rhea’s lips twitched as she not doubly thought of how he burned his way out of the monastery, infants in tow, many many years ago. The two shared a strained smile, before Rhea dismissed the knight and sat at her desk. She rubbed her nose and sighed, letting the stress from watching Byleth and his sister train slowly slip away.

Rhea couldn’t tell which she would prefer: the twins fighting each other, or the twins practicing such dangerous moves.

**Author's Note:**

> This was so much fun to write omg. The three "Ashen Demon" moves are all based on moves in fighting video games with little twists. The male twin Byleth's move is probably obvious, but I will be surprised if you guys recognize the female twin Byleth's move (which I hardly changed from the original). The twin's synchronized move might be easier to spot than the solo female twin's move.


End file.
